In the manual focusing of a digital camera, a photographer searches for a focus position (in-focus position) where a subject becomes sharpest on a monitoring image by adjusting the focus position of a lens, thereby performing a focusing operation. An assist function called peaking is proposed to improve the accuracy of this focusing (see, for example, Patent Literature 1). The peaking function is a function of determining that a portion where the energy of a high frequency component exceeds a threshold in an input image is in focus and of performing highlighting by rendering a marker and the like on a pixel determined to be in focus. The photographer adjusts a focusing position so that most of the markers are rendered on a subject that is intended to be in-focus, thereby achieving the in-focus state.